1. Field of the Invention
The present invention relates to an easy-sintering silicon nitride powder providing a high strength and very reliable silicon nitride-based sintered body. The present invention also relates to a silicon nitride powder which can be used for easily preparing a low viscosity silicon nitride-containing slurry as a starting material for commercially producing a silicon nitride-based sintered body. Silicon nitride-based sintered bodies, one of structural ceramics, are widely used as cutting tips, engine parts, gas turbine components, nozzles for casting, bearing parts, etc. since the bodies are excellent in mechanical properties such as strength, toughness and thermal shock resistance and in other characteristics such as heat resistance and chemical resistance.
2. Description of the Related Art
Known production methods of silicon nitride powder for use as a starting material for a silicon nitride-based sintered body include (1) direct nitridation of metallic silicon, (2) carbothermal reduction and nitridation of silica and (3) thermal decomposition of silicon diimide. These methods have been improved and can provide sintered bodies having enhanced mechanical properties, heat resistance and the like and, recently, can now produce bodies having high purity, a super fine particle size, a high .alpha.-phase content, etc. and having a metallic impurity content, an oxygen content, a particle size and a specific surface area and like which are similar.
It is generally known that properties such as mechanical characteristics and heat resistance of silicon nitride-based sintered bodies are greatly influenced by coarse particles such as coarse primary particles, agglomerated particles and/or fused particles present in the silicon nitride powder as the starting material. The powder prepared in accordance with the method (1) contains a large number of coarse primary particles which were not ground in the milling process of the coarse lumps formed by the direct nitridation reaction, the powder prepared in accordance with the method (2) contains a large number of coarse particles by fusion of the starting silica particles and the powder prepared in accordance with the method (3) contains a large number of coarse particles by agglomeration of fine particles.
The content of the coarse particles such as coarse primary particles, agglomerated particles and/or fused particles present in the silicon nitride powder has been qualitatively measured by observing the particle size and state of the primary particles through a scanning electron microscopic photograph or transmission electron photomicrograph or quantitatively measured by particle size distribution analysis through centrifugal sedimentation method light scattering method, a laser diffraction method, a Coulter counter method, etc. and particle size distribution analysis by a screening method, etc.
The measurement of the coarse particles through the scanning or transmission electron photomicrograph lacks reproducibility due to the small area of the measurement and it is difficult to determine the particle size distribution of agglomerated particles and, thus, a quantitative measurement. The measurement of the coarse particles by any particle size distribution analyzer is on the particle size of the secondary particles and the measuring limit thereof is on the order of %, which make it difficult to reproducibly measure the number of coarse particles in an amount of 0.1% or less.
It is known that the mechanical properties, and other characteristics such as heat resistance and chemical resistance, of a silicon nitride-based sintered body are greatly influenced by coarse particles and foreign particles in the silicon nitride powder. However, a method for measuring the number of coarse particles has not been proposed and quantitative analysis of the influence of the number of coarse particles on various properties of the sintered body has not been reported. This is because there are problems in that a method of quantitative measuring the number of coarse particles has not been established and, therefore, it has not been possible to prepare an easily-sinterable silicon nitride powder with a reduced number of coarse particles which would reliably produce a high strength sintered body.
The silicon nitride sintered body is typically produced by forming a desired shaped body of a silicon nitride powder followed by sintering the same.
Particularly in the commercial production of silicon nitride sintered bodies, the forming process comprises a step of preparing a slurry of the starting powder and it is critical to provide a low viscosity silicon nitride powder-containing slurry.
The known method for providing a shaped body of a silicon nitride powder includes rubber pressing, die pressing, hot pressing, extrusion, injection molding, slip casting, etc. For example, the rubber pressing and die pressing use flowable spray dried granules as the starting material for shaping. If the granules low in flowability are used, the shaped body is non-uniform or the density distribution is not uniform and thus the shrinkage during sintering becomes non-uniform to cause deformation and cracking.
The preparation of granules by spray drying requires an adequate slurry flowability range and the usual slurry viscosity is in a range of 50 to 500 cP, preferably 100 to 400 cP. If the viscosity of the slurry is to high, the slurry supplied to an atomizer of a dryer may disadvantageously block a supply pipe or the outlet for blowing the slurry. If the viscosity of the slurry is to low, the coarse components in the slurry deposit while being supplied to make the composition of the slurry non-uniform and, further, the obtained pellets are too small and have low the flowability.
Accordingly, if the viscosity of a slurry is too high, solvent is added to reduce the ratio of the powder to the solvent in volume and to adjust the viscosity. If the viscosity of a slurry is too low, powder is added to increase the ratio of the powder to the solvent in volume and to adjust the viscosity.
On the other hand, if a slurry with a low powder ratio is dried with spray, the resultant granules may have decents and may even have the form of a doughnut, lowering the flowability. Also, a slurry with a low powder ratio requires a large amount of heat to evaporate the solvent, which lowers the productivity.
Thus, it is necessary that a slurry with as high a powder ratio as possible be prepared and spray dried to form spherical granules with high flowability.
Also, slurry for slip-casting must have an adequate viscosity, typically 50 to 500 cP, preferably 100 to 400 cP. If the viscosity of the slurry is too high, it is difficult for the slurry to conform to intricate portions of the mold, discharging the slurry is difficult, and cracks may appear during drying. If the viscosity is too low, coarse components in the slurry may precipitate during the deposition step, causing a non-uniform composition of the cast, and the slurry may run out from a slit in the mold.
Accordingly, if the viscosity of the slurry is high, a solvent is added to lower the powder ratio and adjust the viscosity. If the viscosity of the slurry is low, a powder is added to increase the powder ratio and adjust the viscosity.
Slip-casting with a slurry having a low powder ratio results in a slow rate of deposition and a long time of shaping, which is not favorable for productivity. Also, it causes an increase in the shrinkage of the cast during drying and sintering, causing cracking.
Thus, it is generally preferable to prepare a slurry with as high a powder ratio as possible.
An excellent shaped body can be easily obtained with a slurry having a high ratio of powder to solvent and a low viscosity. However, since a slurry for shaping silicon nitride ceramics contains a binder for holding the shape of the formed body such as polyvinyl alcohol, polyethylene glycol or acrylate-based copolymer resin, the viscosity becomes high so that it is difficult to adjust the viscosity of the slurry to a preferable value. As a result, conventionally, there is a problem that although the amount of the binder should be increased to obtain a precise shape, the increased binder results in an increase in the viscosity of the slurry containing a powder with a specific surface area of 6 to 25 m.sup.2 /g, making shaping operation difficult.
The object of the present invention is to develop a method for measuring the amount of coarse particles in silicon nitride powder and, taking advantage of the developed method, to provide an easy-sintering silicon nitride powder reliably giving a sintered body with a high strength and little fluctuation in characteristics.
Another object of the present invention is to provide a silicon nitride powder which gives a silicon nitride-containing slurry with a low viscosity and a high powder concentration even when a binder is added thereto for shaping.